1. Field of the Invention
The present invention relates to a gap measurement apparatus for use in laser beam machines, for measuring a gap between a nozzle and a workpiece.
2. Description of Related Art
A related art gap measurement apparatus for use in laser beam machines sometimes cannot measure a gap between a nozzle and a workpiece correctly since plasma occurs between the nozzle and the workpiece and acts as an impedance because of a lack of laser power during machining of the workpiece. Therefore, if a change in the capacitance which is equal to or larger than a reference value is detected when the plasma occurs during the machining of the workpiece, the gap between the nozzle and the workpiece is maintained for a fixed time period in order to prevent the nozzle from being moved to collide with the workpiece due to erroneous measurement of the gap between the nozzle and the workpiece (refer to, for example, patent reference 1).
On the other hand, a related art gap measurement apparatus disclosed by, for example, patent reference 2 can measure a voltage which appears at a sensor electrode to determine the real part and the imaginary part of the measured voltage in order to measure a gap between a nozzle and a workpiece even if plasma occurs between the nozzle and the workpiece. The related art gap measurement apparatus disclosed by patent reference 2 includes a single signal generator to generate an AC signal of one frequency, and supplies the AC signal to a cable connected to the nozzle. The related art gap measurement apparatus sets the frequency of the AC signal so that the plasma acts as a pure ohmic resistance.    [Patent reference 1] JP,6-7976,A (see paragraph number [0008] and FIG. 1)    [Patent reference 2] JP,2000-234903,A (see paragraph number [0010] and FIG. 1)
While related art gap measurement apparatus constructed as mentioned above can measure the gap between the nozzle and the workpiece even when plasma occurs between the nozzle and the workpiece, a problem with them is that since the AC signal supplied to the cable connected to the nozzle has only one frequency, the accuracy of the measurement of the gap is reduced when plasma occurs in the gap to be measured.